Letterbox Hybrids became a geocache type early 2001 my sources tell me and there are still some out there to find from that year. So that means there have been letterbox hybrids placed in 14 separate years since caching began. Your challenge is find letterboxes placed in 8 different years. Any eight different years of the 14 possible will qualify you for this challenge.
We personally have found letterboxes from 12 of the 14 years so get out there and find some of these oft forgotten cache types. You can show your qualification as a bookmark list or list them with your log by giving cache name, GC #, and the year hidden.
Good Luck